1. Technical Field
The present invention relates to a resin composition having excellent biodegradability and excellent gas barrier properties, and a film or a shaped (or molded) article comprising such a resin composition.
2. Background Art
A film having a gas barrier property (gas barrier film) is a kind of functional film that has been widely used in the fields of food, medicine, agricultural chemicals, cosmetics, etc., wherein contents to be contained therein are stored or protected while the "quality" of the contents is liable to cause a problem. One of such important use of the film includes the field of "packaging".
Packaging includes making or putting an object into a package or wrap. The material for the packaging is desired to have a wide variety of functions. For example, packaging may serve various functions including providing mechanical protection, safety, sanitary protection, workability, adaptability to goods (transparency, printability, heat sealing property), utility, profitability, etc. Having a "gas barrier property" against various gases is important as it affects the storability of the packaged contents such as food. Along with recent diversification in the form of goods distribution, in packaging techniques, and an intensification of additive control, change in taste, etc., the gas barrier property has become increasingly more important. On the other hand, the gas barrier property has heretofore been a serious weak point of ordinary plastic materials.
Factors which can deteriorate a food include oxygen, light, heat and/or moisture. Among these factors, oxygen has been considered to be a substance causing such deterioration. A material having a gas barrier property (gas barrier material) is a material which has a main function of effectively intercepting oxygen. Such a gas barrier material exhibits the function of intercepting oxygen, and simultaneously exhibits a function which is essential for various measures for controlling the deterioration of food (such as gas charging and vacuum packaging). The gas barrier material has been utilized very effectively in many fields such as food packaging inclusive of confectionery bags, bags for dried bonito, pouches for retorted foods, containers for carbonated drinks, etc., or packaging for cosmetics, agricultural chemicals, and medical use, on the basis of its barrier function to various kinds of gases such as oxygen, organic solvent vapors, aromas; or on the basis of its function of preventing corrosion, odor, sublimation, etc., based on the barrier function thereof.
Among films comprised of a thermoplastic resin, those films comprising oriented polypropylene, polyester, polyamide, etc., particularly have excellent mechanical, heat resistance, and transparency properties, etc., and therefore these films are widely used as a packaging material. However, the barrier property against oxygen permeability is insufficient as these films have large gas permeability. Therefore, in a case where a film comprised of such a material is used for food packaging, the food in the package is liable to be deteriorated due to degradation based on oxidation, or the function of aerobic bacteria, etc. Accordingly, when a film of the above-mentioned material such as polypropylene is used for food packaging, it is usual to adopt a method wherein another film (or layer) having an excellent oxygen barrier property is laminated onto the film of the above-mentioned material.
The following two methods are representative measures for forming such a film having a good barrier property to oxygen. One method involves using a metal foil lamination such as aluminum foil lamination. The other method involves vapor-depositing a metal onto a surface of a thermoplastic resin film. In these methods, the excellent gas barrier property (particularly, a gas barrier permeability to oxygen) of the resultant metal foil has been effectively utilized.
On the other hand, as a transparent plastic raw material having a small oxygen permeability, there have heretofore been known some films comprising a raw material such as polyvinyl alcohol, polyethylene-vinyl alcohol copolymer, and polyvinylidene chloride-type resin. However, these plastic materials have an oxygen permeability to a certain degree which is never negligible, while a metal or glass raw material to be used for canned foods or bottled foods only has a substantially no oxygen permeability.
Methods involving dispersing an inorganic substance (in a flat shape) in a resin are known techniques for imparting gas barrier properties or increasing the gas barrier property of a resin. For example, Japanese Laid-Open Patent Application (KOKAI) No. 148532/1987 (i.e., Sho 62-148532) describes a process for producing a film having a gas barrier property wherein a coating liquid composition comprising a transparent thermoplastic resin and mica in the form of flakes having a particle size of 500 .mu.m or smaller and an aspect ratio of 5 or larger, is applied onto a base material having a releasing property and then dried, and thereafter is peeled from the base material.
In addition, Japanese Laid-Open Patent Application No. 043554/1989 (i.e., Sho 64-043554) discloses a resin composition for barrier layer wherein muscovite (KAl.sub.3 (AlSi.sub.3 O.sub.10)(OH).sub.2 ; non-swelling type mica) is used as mica in the form of flakes, the particle size is made smaller than 325 mesh (44 .mu.m), and the aspect ratio is in the range of 20-140. Further, Japanese Laid-Open Patent Application No. 093542/1991 (i.e., Hei 03-093542) discloses a process for producing a plastic film having a barrier property wherein a coating composition comprising a silyl group-containing modified polyvinyl alcohol and synthetic hectorite (trade name: Laponite XLS, mfd. by Nihon Silica Kogyo Co.) in a wt. ratio of 50:50, is applied onto a biaxially oriented polyethylene terephthalate (OPET), and then subjected to drying and heat treatment (130.degree. C. to 150.degree. C.).
In general, the above-mentioned packaging material is used for a "disposable or throwaway" application. Therefore, it is important to impart a biodegradability to the packaging material, as some problems in the terrestrial environment have attracted more and more attention in recent years.
However, at present, a material which has a high gas barrier property to an extent such that it is suitable for the preservation and/or protection of a content to be contained in the material, and also has a biodegradability to an extent such that it can contribute to the maintenance or protection of the terrestrial environment has not yet been on the market.